Usually vehicles the prime mover of which is actuated by fuel are equipped with a fuel gauge for indicating the amount of fuel contained in the fuel tank. For instance, a fuel gauge for an automotive vehicle has a movable pointer for indicating the amount of fuel contained in the fuel tank by measuring the fuel level of the residual fuel. With this conventional fuel gauge, the vehicle driver can only estimate the amount of fuel remaining in the fuel tank. When the pointer of the fuel gauge is around a position indicating that the fuel tank is empty, the vehicle driver has to guess the distance that the vehicle can travel with the residual fuel.
Recently, various fuel consumption meters are provided for indicating the fuel consumption rate of the engine of the vehicle. However, such a fuel consumption rate meter does not satisfy the vehicle driver since it does not indicate how long the vehicle can travel with the residual fuel.
Further, an apparatus for indicating the distance that the vehicle can travel with the residual fuel is proposed in a Japanese patent pre-publication (publication before the examination) No. 51-54140. In accordance with the apparatus disclosed in the pre-publication, a predetermined amount of fuel corresponding to the capacity of the fuel tank is preset when the fuel tank is full, and the residual amount of fuel estimated by the preset amount of fuel and a fuel consumption rate of the engine of the vehicle is divided by the fuel consumption rate. However, the fuel consumption rate is not actually measured but a fixed value selected out of three (high, medium and low) values is used wherein the vehicle driver is supposed to select one fixed value via estimation of the operating condition of the engine and the vehicle.
Therefore, it will be seen that the accuracy of the distance detected by the division is not high since the actual fuel consumption rate varies radically in accordance with the way of driving of the vehicle, gear position, changes of load, air temperature, etc. Further, in the apparatus disclosed in the pre-publication, since the amount of fuel is to be preset with a predetermined value, the vehicle driver has to operate a key or a switch to preset the same whenever the fuel tank of the vehicle is filled with fuel. Further, the above-mentioned apparatus does not include a memory to store the amount of residual fuel remaining in the fuel tank, and thus the apparatus does not work after the power supply (usually controlled by the ignition key) is turned off, until the predetermined value corresponding to the capacity of the fuel tank is preset again.